The present invention relates in general to subsea wellheads for oil and gas wells, and in particular to a nested stack-down casing hanger configuration which allows the pressure in the intermediate casing annuli to be monitored without penetrating the outer pressure containing housing or casing walls which separate the annuli from the external environment. Although the present invention has particular utility with respect to subsea wells, the invention is also applicable to land and offshore surface drilled wells.
In order to conform to various regulations and to protect life, property, and the environment, it is common practice on surface drilled wells to monitor the pressure in the various casing annuli for sustained casing head pressure (SCP). Pressure containing side outlets are provided in the casing and tubing heads, through which the annulus pressure can be measured. However, because such side outlets themselves create potential leak points, and because of the difficulty in detecting leaks, side penetrations in subsea wellhead housings are usually avoided. Exceptions are made in the regulations for high pressure subsea wells, such that it is required only to monitor pressure in the production annulus. In fact, such body penetrations are actually prohibited by some regulations. In any event, body penetrations in subsea wellheads could create potential hazards greater than those originally addressed by annulus monitoring.
Despite the difficulties inherent in monitoring annulus pressure in subsea wells, regulations have been proposed which would require that the pressure be monitored in every annulus in the well. Thus there is a need for a method of monitoring annulus pressure which does not require penetration of the pressure containing casings or housings. Even in the absence of such regulations, such a method would be most useful and desirable. Several prior art methods for monitoring annulus pressure in subsea wells are described in U.S. Pat. Nos. 5,544,707 and 4,887,672. A more complete discussion of the various regulations and the state of the prior art with respect to annulus pressure monitoring is presented in copending U.S. patent application Ser. No. 09/776,065, which is commonly owned herewith and the entirety of which is hereby incorporated by reference for all purposes.
Typical prior art wellhead systems have utilized a xe2x80x9cstack-upxe2x80x9d casing hanger configuration. In this type of system, the hanger for each successively smaller diameter casing string is landed on top of the hanger for the next larger casing string. Each hanger is locked and sealed to the wellhead housing bore above the next lower hanger. Thus, as each hanger is installed in the wellhead housing, the next lower hanger (and the associated annulus) becomes inaccessible.
For the purposes of illustration, a typical stack-up subsea wellhead system is shown in FIG. 1. The wellhead system comprises a conductor housing 12 attached atop conductor pipe 18 and locked into permanent guide base 10. The wellhead housing 14 is landed in the conductor housing 12 and includes wellhead bore 16. Second intermediate casing hanger 32 is landed in the wellhead housing 14 and supports second intermediate casing string 42. Hanger 32 is provided with annulus access port 36, which allows for fluid communication between the wellhead bore 16 and the xe2x80x9cCxe2x80x9d annulus 50 after installation of hanger 32. After the hanger 32 is landed in the wellhead housing 14, pack-off 34 is installed between hanger 32 and the wellhead housing 14, preventing further communication with access port 36.
First intermediate casing hanger 26 is then landed atop second intermediate casing hanger 32 and supports first intermediate casing string 40. Hanger 26 is provided with annulus access port 30, which allows for fluid communication between the wellhead bore 16 and the xe2x80x9cBxe2x80x9d annulus 48 after installation of hanger 26. After the hanger 26 is landed on hanger 32, pack-off 28 is installed between hanger 26 and the wellhead housing 14, preventing further communication with access port 30.
Production casing hanger 20 is then landed atop first intermediate casing hanger 26 and supports production casing string 38. Hanger 20 is provided with annulus access port 24, which allows for fluid communication between the wellhead bore 16 and the production or xe2x80x9cAxe2x80x9d annulus 46 after installation of hanger 20. The xe2x80x9cAxe2x80x9d annulus is located between the production casing string 38 and the production tubing, shown in phantom at 44. After the hanger 20 is landed on hanger 26, pack-off 22 is installed between hanger 20 and the wellhead housing 14, preventing further communication with access port 24. As is apparent from the figure, once all the casing hangers have been installed in the wellhead housing 14, access to the xe2x80x9cBxe2x80x9d and xe2x80x9cCxe2x80x9d annuli is prevented.
In accordance with the present invention, these and other disadvantages in the prior art are overcome by providing a wellhead system which comprises a wellhead housing and a plurality of concentric casing strings, each of which is suspended from a corresponding casing hanger. The casing hanger for the radially outermost casing string is supported in said wellhead housing and the casing hanger for each successively smaller casing string is supported in the casing hanger for the next radially larger casing string. Each casing string defines a corresponding annulus which surrounds said casing string and is located below the casing hanger for said casing string. Furthermore, at least one casing hanger comprises a bypass port or similar means for providing fluid communication between the annulus below said casing hanger and an area above said casing hanger.
Thus, the wellhead system of the present invention comprises a xe2x80x9cstack-downxe2x80x9d casing hanger configuration. In this type of system, the hanger for each successively smaller diameter casing string is landed or xe2x80x9cnestedxe2x80x9d within the hanger for the next larger casing string. This approach allows the pack-off for each casing hanger to be retrieved independently, thus allowing fluid communication to be established with any of the casing annuli after all of the casing strings and hangers have been installed. Thus the pressure in each annulus may be monitored while the well is in production mode.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments.